Troy-Bilt 78 - Manual

IV For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com

Introduction 1 Professional Service Manual Intent This manual is intended to provide service dealers with an introduction to proven diagnostic and repair proce- dures for 78/83/90 series MTD horizontal shaft engines. Disclaimer : The information contained in this manual is correct at the time of wri...

Introduction 3 Fasteners • Most of the fasteners used on the MTD engine are metric. Some are fractional inches. For this reason, wrench sizes are frequently identified in the text, and measurements are given in U.S. and metric scales. • If a fastener has a locking feature that has worn, replace the ...

78/83/90 Series Horizontal Shaft Engines 4 Model and serial number The model and serial number can be found on a white sticker with a bar code. The sticker is located on the right side of the engine at the bottom of the block. See Figure 1.1. NOTE: The serial number will always start with the model ...

Introduction 5 Maintenance The information in this manual applies to the MTD engine. Some basic principles may apply to engines pro-duced by other manufacturers. As the saying goes “an ounce of prevention is worth a pound of cure”. The same can be said about preventive maintenance on outdoor power e...

Introduction 7 Oil type and capacity MTD engines use oil with a SF/SG API rating or better is the recommended oil for this engine. The winter engines use SAE 5W-30 oil and the summer engines use SAE 10W-30 oil. The oil capacity for all of the 78/83/90 series engines is 37 fl.oz (1.1 liters). • Check...

78/83/90 Series Horizontal Shaft Engines 8 To check the oil with a threaded dip stick: 1. Twist and remove the dip stick from the engine. 2. Clean the oil off of the tip of the dipstick. 3. Re-insert the dipstick without threading it in to get the oil level reading. See Figure 1.5. 4. The oil level ...

Introduction 9 Fuel Gasolines currently on the market are not pure gasoline. Today’s fuels have alcohol and other additives in them to reduce emissions. The fuel make up can vary seasonally and geographically. Fuel with alcohol added to it is sometimes referred to as “oxygenated fuel”. The extra oxy...

Introduction 11 Valve lash Valve lash is the clearance between the top of the valve stem and the rocker arm. The valve lash should be checked after the first 25 hours of use and every 100 hours after that. Valve lash can be checked and adjusted using the following steps: 1. If the engine has been ru...

Introduction 13 13. Double-check the clearance after tightening the jam nut, to confirm that it did not shift. Re-adjust if necessary. 14. Rotate the engine through several compression cycles: • Observe the movement of the valve gear. • Return the piston to TDC compression stroke and re-check the va...

78/83/90 Series Horizontal Shaft Engines 14 General torque specifications Maintenance Chart 0 0 0 H ] L V 0 0 0 0 H ] L V V E O W I V E O Q L H G D U * P 1 P 1 V E O W I V E O Q L P 1 P 1 V E O W I V E O Q L P 1 P 1 V E O W I V E O Q L P 1 P 1 V E O W I V E O Q L P 1 P 1 V E O W I V E O Q L P 1 P 1 ...

Introduction 15 Frequently used specifications Minimum Maximum in. mm in. mm Intake valve lash 78 0.004 0.10 0.006 0.15 83 0.004 0.10 0.006 0.15 90 0.004 0.10 0.006 0.15 Exhaust valve lash 78 0.006 0.15 0.008 0.20 83 0.006 0.15 0.008 0.20 90 0.006 0.15 0.008 0.20 Spark plug gap 78 0.026 0.65 0.030 0...

B ASIC T ROUBLESHOOTING 17 Definitions Troubleshooting - The act of gathering information by preforming tests and direct observations. Diagnosis - Developing and testing theories of what the problem is, based on the information gathered in trouble-shooting. Introduction Diagnosing an engine is an ar...

78/83/90 Series Horizontal Shaft Engines 18 IV. Unusual exhaust tone There are tools that the technician can use in order to define the problem, such as: 1. Interview the customer. 1a. Get a good description of their complaint. 1b. If it is an intermittent problem, verify what conditions aggravate t...

B ASIC T ROUBLESHOOTING 23 Repairing the problem The third step in the troubleshooting process is to repair the problem. This step consists of: A. Form a diagnosis by using all of the information gathered from the troubleshooting that was performed. B. Physically perform the repair. The fourth, and ...

78/83/90 Series Horizontal Shaft Engines 24 Prime test To perform a prime test: 1. Prime the engine through the carburetor throat using a squirt bottle, filled with clean fresh gasoline. 2. Make sure the throttle is in the run position and the safety key if fully inserted. 3. Attempt to start the en...

B ASIC T ROUBLESHOOTING 25 7. Compare the results to the following chart. Leak-down Testing Results Symptom Possible cause Air escaping from the breather Worn cylinder or piston rings. Possible blown head gasket Air escaping from the exhaust Leaking exhaust valve Air escaping from the carburetor Lea...

78/83/90 Series Horizontal Shaft Engines 26 Compression test To perform a compression test: NOTE: Compression should be in the range of 40-75 PSI (2.8-5.2 Bar). • Disconnect the high-tension lead from the spark plug and ground it well away from the spark plug hole. • Remove the spark plug using a 13...

B ASIC T ROUBLESHOOTING 27 PCV testing The PCV (Positive Crankcase Ventilation) valve is located in the valve cover and allows the crankcase pressure to escape. Leakage and blockage are the two failure modes for a PCV system. Either mode will cause crankcase pressure to build-up, though the effects ...

A IR I NTAKE S YSTEMS 29 MTD builds horizontal crank engines for snow blowers and chore performers. The differences between snow engines and chore engines are the muffler and the air intake system. Therefore the air intake system for the snow and chore engines will be discussed separately, as will t...

A IR I NTAKE S YSTEMS 31 5. Install the air filter by following the previous steps in reverse order. NOTE: When installing the air filter, the hole in the bottom of the paper element must fit over the riser in the air filter base. See Figure 3.6. Figure 3.6 Riser Air filter base For Parts Call 606-6...

78/83/90 Series Horizontal Shaft Engines 32 Air filter base and control panel (summer engines) To remove/replace the air filter base and control panel: 1. Remove the air filter following the steps described in the previous section. 2. Remove the three screws that hold the air filter base to the inta...

A IR I NTAKE S YSTEMS 33 NOTE: All summer engines built on or after January 1, 2011 will have a hose from the evaporative emis-sions (EVAP) system attached to the control panel above the breather hose. See Figure 3.10. 9. Pull the control panel straight off the front of the engine. 10. Install the c...

78/83/90 Series Horizontal Shaft Engines 34 Carburetor and Insulator 1. Siphon the fuel out of the fuel tank. NOTE: Dispose of drained fuel in a safe and responsible manner. 2. Remove the fuel tank front bezel using a 10 mm wrench. See Figure 3.11. NOTE: The bezel may get stuck on the cup for the re...

A IR I NTAKE S YSTEMS 35 4. Remove the Heat box (snow engines) or the control panel (summer engines) by following the steps described in appropriate section of this chapter. 5. Disconnect the throttle linkage and spring. See Figure 3.13. 6. Slide the carburetor off of the mounting studs. 7. Remove t...

F UEL S YSTEM AND G OVERNOR 37 The function of the fuel system is to store fuel, mix the fuel with air in the correct ratio and deliver it to the intake port. The fuel system consists of the following components: • Fuel tank • Fuel lines • Fuel filter • Carburetor NOTE: When working on the fuel syst...

78/83/90 Series Horizontal Shaft Engines 38 Test fuel for alcohol Fuels currently on the market contain a wide array of additives. Some of these additives oxygenate the fuel. Oxy- genated fuel reduces emissions, and is required in some parts of the United States. Fuel make-up varies seasonally and g...

F UEL S YSTEM AND G OVERNOR 39 Fuel tank vent The fuel tank vent performs the important task of allowing air into the fuel tank. As fuel is being used by the engine, the fuel level in the tank drops. The dropping fuel level then creates a vacuum in the tank. If the fuel tank could not draw air throu...

78/83/90 Series Horizontal Shaft Engines 40 The fuel filter The fuel filter is located in the fuel tank nipple. It can be removed and cleaned with a can of carb cleaner or it can be replaced. See Figure 4.4. NOTE: If cleaning a filter, back-flush it by spraying the carb cleaner through the barb end ...

F UEL S YSTEM AND G OVERNOR 41 The fuel tank To remove the fuel tank: 1. Drain the tank. 2. Remove the four screws that secure the fuel tank using a 12 mm wrench. See Figure 4.5. 3. Lift up on the fuel tank and disconnect the fuel line. See Figure 4.6. NOTE: On engines equipped with an EVAP system, ...

78/83/90 Series Horizontal Shaft Engines 44 Troubleshooting the EVAP System There are four failure modes for the EVAP system: • A leak in the system - will allow dirt ingestion. • A blockage in the system (between the canister and the fuel tank) - acts a plugged vent. • A blockage in the system (bet...

F UEL S YSTEM AND G OVERNOR 45 Testing the roll over valve The roll over valve vent has two functions. The first function is to vent the tank and the second function is to close off the vent if the tank is inverted. Test the roll over valve by: 1. Disconnect the hose that runs from the charcoal cani...

78/83/90 Series Horizontal Shaft Engines 46 Charcoal canister To remove/replace the charcoal canister: 1. Disconnect the hoses from the canister. See Figure 4.15. 2. Remove the screw from the canister mounting strap using a 10 mm wrench. See Figure 4.16. To Install the canister: 3. Insert the tab of...

F UEL S YSTEM AND G OVERNOR 47 Roll over valve To remove the roll over valve: 1. Gently pry the grommet from in between the roll over valve and the fuel tank. See Figure 4.18. NOTE: Leaving the hose connected to the roll over valve will help prevent it from falling inside the tank once the grommet i...

78/83/90 Series Horizontal Shaft Engines 48 Choke 78/83/90 series MTD engines are equipped with a choke. The snow engines are also equipped with a primer, both of which must be used to start the engine. NOTE: The choke should be opened after the engine starts. This can be a source of starting issues...

F UEL S YSTEM AND G OVERNOR 49 The choke rod is part of the heat box assembly mounted on the front of the carburetor. See Figure 4.22. NOTE: The choke rod can be bent slightly to facilitate adjustment. To adjust the choke rod: 1. Remove the choke knob and the engine shroud by following the steps des...

78/83/90 Series Horizontal Shaft Engines 52 Disassembly and rebuilding of the carburetor 1. Clamp-off the fuel line to prevent fuel spillage, then disconnect the fuel line from the carburetor. 2. Disconnect the primer hose. 3. Remove the carburetor by following the steps described in Chapter 3: Air ...

F UEL S YSTEM AND G OVERNOR 55 12. Examine the metering plug: See Figure 4.33. • Fuel, drawn from the central column via the long fuel feed leg, is metered by the brass orifice in the tip of the metering plug. • Air, drawn from the emulsion air port, is metered by the size of the brass orifice at th...

F UEL S YSTEM AND G OVERNOR 57 6.Remove the fuel shut-off lever. See Figure 4.38. NOTE: There is a passage cast into the shut-off lever. When this passage lines up with the two posts in the carburetor, fuel will flow to the needle valve. 7. Remove the rubber gasket. See Figure 4.40. 8. Re-assemble t...

78/83/90 Series Horizontal Shaft Engines 58 Governor The engine speed is controlled by a balance between the force applied by a spring (pulling the throttle open) and a flyweight mechanism within the engine applying force to the governor arm (pushing the throttle closed). See Figure 4.41. NOTE: Whil...

F UEL S YSTEM AND G OVERNOR 59 Governor shaft To remove or replace the governor shaft: 1. Remove the engine from the equipment that it pow-ers. 2. Remove the governor arm by following the previously described steps. 3. Remove the flywheel by following the steps described in Chapter 7: Ignition Syste...

78/83/90 Series Horizontal Shaft Engines 60 Governor cup and the governor gear The Governor gear and cup are located inside the crankcase cover. The flyweights and the governor cup are inter locked on this family of engines. The governor gear and cup are serviced as a complete assembly. 1. Remove th...

F UEL S YSTEM AND G OVERNOR 61 Governor adjustment To adjust the governor: 1. Remove the fuel tank by following the procedures described in the fuel tank section of this chapter. 2. Loosen the governor arm nut but do not remove the nut completely. 3. Pry open the governor arm crimp with a flat head ...

Lubrication 63 Oil type and quantity MTD engines use oil with a SF/SG API rating or better is the recommended oil for this engine. • The winter engines use SAE 5W-30 oil. • The summer engines use SAE 10W-30 oil. • The oil capacity for all of the 78/83/90 series engines is 37 fl.oz (1.1 liters). • If...

78/83/90 Series Horizontal Shaft Engines 64 Oil dip stick NOTE: There are two types of dip sticks that can be found on MTD engines; a threaded dip stick that was used on older engines and a quarter turn dip stick that is used on engines currently being produced. See Figure 5.1. To check the oil with...

Lubrication 65 Dip stick tube removal To remove/replace the dip stick tube: NOTE: The procedure to remove the dip stick tube is the same for both styles of dip stick. NOTE: A threaded dip stick and tube assembly is inter- changeable with a quarter turn dip stick and tube assembly. 1. Remove the uppe...

78/83/90 Series Horizontal Shaft Engines 66 Lubrication system MTD uses a splash lube system for its horizontal shaft engines. The connecting rod has a dipper on it that “splashes” oil around the inside of the engine. See Figure 5.5. NOTE: The cam and the balance shaft were removed for a better view...

Lubrication 67 Positive crankcase ventilation valve To remove the valve cover and PCV valve: NOTE: The PCV valve is located inside the valve cover. The function and test procedures for the PCV valve is covered in Chapter 2: Basic Troubleshoot-ing. 1. Disconnect and ground the spark plug wire. 2. Squ...

Starter and Charging System 69 There are two starter systems available for the MTD engine. The first one is a recoil starter and the second is a 110 volt electric starter. All of the 78/83/90 series MTD engines equipped with an electric starter, are also equipped with a recoil starter. Recoil Starte...

78/83/90 Series Horizontal Shaft Engines 70 Starter Cup The starter cup is a steel cup that is bolted to the flywheel. 1. Inspect the inside of the starter cup. See Figure 6.2. NOTE: If the starter was failing to engage the fly- wheel, and the edges of the slots inside the cup are burred or damaged,...

Starter and Charging System 71 Starter Rope The most common failure mode for most recoil assemblies is a broken rope. NOTE: If the spring was not damaged when the recoil sprung back, it is possible to simply remove the rem- nants of the old rope and install a new rope. 1. Remove the starter by follo...

Starter and Charging System 73 Starter pulley and recoil spring If damage is suspected, the recoil may be disassembled by: NOTE: The recoil spring is nested within the starter pulley and both parts are sold as a single part number. 1. Remove the starter by following the steps described earlier in th...

Starter and Charging System 75 Electric starter The electric starter is only available on the snow engine. It is powered by an extension cord that is plugged into household 120 volt AC current. The starter and switch assembly are one piece and are not serviceable. To replace the starter assembly: 1....

Starter and Charging System 77 Charging system Some engines are equipped with a charging system. The charging system used on MTD engines consists of three components, the rotor, stator and the rectifier. • Alternator rotor: The rotor consists of five magnets on the inside of the flywheel that rotate...

78/83/90 Series Horizontal Shaft Engines 78 Charging system testing To test the charging system: 1. Locate the charger harness. It will be by the right handle bar of the snow thrower when the engine is installed. See Figure 6.16. 2. Start the engine and run it at full throttle. 3. Connect the black ...

Starter and Charging System 79 Stator To remove/replace the stator: 1. Remove and ground the spark plug wire. 2. Remove the flywheel by following the steps described in Chapter 7: Ignition System. 3. Remove the baffle that covers the charger harness using a 10mm wrench. See Figure 6.19. 4. Slide the...

Ignition System 81 Troubleshooting the ignition system The purpose of the ignition system is to provide a spark in the combustion chamber at the proper time to efficiently ignite the fuel/air mixture. The steps in troubleshooting the ignition system are: 1. Examine the spark plug(s) by following the...

78/83/90 Series Horizontal Shaft Engines 82 Stop switch All MTD horizontal engines that are in use in North America have a stop switch built into the throttle lever assembly. MTD engines used on snow blowers have an additional stop (ignition) switch in the engine shroud. To test the stop switch (thr...

Ignition System 83 Remote (ignition) stop switch (snow engines) To test the remote stop switch: 1. Remove the engine shroud by following the proce- dures described in Chapter 3: Air Intake System. 2. Disconnect the two wires from the remote switch. See Figure 7.5. 3. Connect a digital multimeter to ...

78/83/90 Series Horizontal Shaft Engines 84 Test for ignition that won’t turn off MTD engines are turned off by removing the spark from the engine. This is accomplished by shorting the primary windings of the coil to ground. If the engine does not stop when the key is removed and/or the throttle is ...

Ignition System 85 The module The coil in this ignition system is an inductive discharge magneto, contained in a single module. • The inductive discharge magneto has a two leg design. • The magneto is energized by the passing of a magnet mounted in the flywheel. • Ignition timing is set by the locat...

78/83/90 Series Horizontal Shaft Engines 86 Module removal 1. Unplug the spark plug. 2. Remove the engine shroud by following the steps procedures in Chapter 3: Air Intake Systems. 3. Remove the front fuel tank shroud using a 10 mm wrench. See Figure 7.11. 4. Remove the blower housing. NOTE: The rec...

Ignition System 87 Installing the module and setting the air gap NOTE: If just setting the air gap, loosen the module mounting screws first then follow the same steps as described below. 1. Rotate the flywheel so that the magnets are away from where the module is mounted. 2. Install the module. Do n...

78/83/90 Series Horizontal Shaft Engines 88 Flywheel The flywheel holds the magnets. These magnets induce a field in the module which in turn produces a spark. It also controls the timing of the ignition system by controlling when the magnets are introduced to the module. A sheared flywheel key will...

Ignition System 89 About the spark plug • The spark plug is a F6RTC, gapped to 0.026” - 0.030” (0.65 - 0.75 mm). NOTE: The F6RTC plug is the only plug that is EPA certified for the MTD engine. • Wear rate will vary somewhat with severity of use. If the edges of the center electrode are rounded-off, ...

Exhaust 91 The exhaust system is a frequently overlooked component of an engine. It is important to make sure the muffler is in good condition and free of debris and/or insects. NOTE: A blocked muffler will result in poor performance. If a muffler is completely blocked the engine may not start. Summ...

78/83/90 Series Horizontal Shaft Engines 92 Muffler removal/replacement 1. Remove the two nuts that hold the manifold pipe to the cylinder head using a 13mm wrench. See Figure 8.3. 2. Lift the muffler off of the engine. 3. Clean all of the gasket material off of the cylinder head and the muffler. Se...

Exhaust 95 6. Remove the two muffler nuts using a 13mm wrench and lift the muffler off of the engine. See Figure 8.10. 7. Clean all of the gasket material off of the cylinder head and the muffler (if reusing the muffler) NOTE: The MTD engine uses a graphite exhaust gasket. It is not reusable and mus...

Cylinder head 97 Cylinder head removal The Cylinder head of the MTD engine can be removed without removing the engine from the piece of equipment. To remove the cylinder head: 1. Disconnect and ground the spark plug high tension lead. 2. Remove the spark plug using a 13/16” or 21mm wrench. 3. Rotate...

Cylinder head 99 NOTE: The 490 engine has an over sized bolt with a Belleville washer in the number 4 position. The wrench size and torque is the same as the other three bolts. See Figure 9.6. 14. Lift the cylinder head off of the engine. 15. Carefully clean all sealing surfaces of all gasket resi-d...

78/83/90 Series Horizontal Shaft Engines 100 Cylinder head installation 1. Place a new head gasket on the cylinder, allowing the alignment dowels to hold it in place. See Figure 9.8. NOTE: MTD uses graphite head gaskets that have a bead of silicon on them. They are not reus-able. 2. Position the cyl...

Cylinder head 101 Valves The valves and valve seats can be serviced by grinding and lapping or the head can be replaced. Depending on local machine and labor costs, it is probably more economical to replace the cylinder head versus servicing the valves. To service the valves: NOTE: Servicing valves ...

Cylinder head 103 10. Test the valves for leaks by: 10a. Place the cylinder head on a couple of wood blocks with the valves facing up. 10b. Pour a small amount of gasoline or parts cleaning solvent into the combustion chamber (just enough to cover the valves). 10c. Let the cylinder head sit for ten ...

Crankshaft, piston and connecting rod 105 The exact procedure a technician uses to disassemble an engine depends on the type of repairs needed. This chapter is written as a set of procedures that should provide the user with sufficient information to complete any fea-sible repair to the engine short...

78/83/90 Series Horizontal Shaft Engines 108 Crankshaft inspection 1. Inspect the crankshaft journals and the crank pin for galling, scoring, pitting or any other form of damage. NOTE: This is mostly a visual check. Measurement is to determine if it is within the specifications after it is found to ...

Crankshaft, piston and connecting rod 109 Piston Inspection 1. Clean the piston and remove all carbon from the rings and ring groves. 2. Clean the cylinder bore and remove all carbon. 3. Insert one ring into the cylinder. Push it down about one inch from the top. See Figure 10.8. 4. Measure the end ...

Crankshaft, piston and connecting rod 111 Connecting rod inspection 1. Inspect the connecting rod for cracks or any signs of damage. 2. Install the rod cap and tighten to a torque of 106 - 124 in-lbs (12 - 14Nm). 3. Measure the inside diameter of the connecting rod at both ends and compare the measu...

Crankshaft, piston and connecting rod 113 Reassembly 1. Install the governor shaft. NOTE: The governor shaft MUST be installed before the crankshaft is installed. 2. Clean the cylinder 2a. Remove all gasket material from all mating surfaces. 2b. Clean the cylinder and crank case cover. 3. Oil seals ...

Crankshaft, piston and connecting rod 115 Engine specifications chart Specification New Min New Max Service Limit in. mm in. mm in. mm Bore (digit 2&3 of the model number) 78 3.071 78.02 3.072 78.03 3.080 78.23 83 3.268 83.02 3.269 83.03 3.277 83.24 90 3.544 90.02 3.544 90.03 3.553 90.26 Crank p...

Crankshaft, piston and connecting rod 119 Engine torque values chart Fastener Torque 78 83 90 Blower housing 80-106 in-lbs (9-12 Nm) 80-106 in-lbs (9-12 Nm) 80-106 in-lbs (9-12 Nm) Carburetor drain bolt 80-106 in-lbs (9-12 Nm) 80-106 in-lbs (9-12 Nm) 80-106 in-lbs (9-12 Nm) Carburetor mounting nuts ...

Failure Analysis 121 A properly maintained engine will provide years of service. Occasionally an engine will fail. An important part of working on engines is finding out why they failed. Was it something the customer did? Was it a manufacturing defect? Did the engine just wear out? All of these ques...

Failure Analysis 123 8. Because the oil suspends the abrasive particles, the engine components that are immersed in oil will show definite signs of abrasive ingestion especially around the connecting rod and main bearing journals. See Figure 11.5. NOTE: Abrasives that are trapped in the oil will cau...

78/83/90 Series Horizontal Shaft Engines 124 Insufficient lubrication The bearing surfaces in an engine are not smooth. As a result of the machining processes to make the engine parts, there are little peaks and valleys that are only visible on a microscopic scale. These peaks are called asperi-ties...

Failure Analysis 125 Engine Overspeed The MTD engine is designed for a maximum speed of 3600 rpm. When the governor is unable to control the engine rpm, the engine can accelerate past the safe maximum speed. When an engine runs beyond its designed speed, a few things happen: 1. As the piston moves u...

78/83/90 Series Horizontal Shaft Engines 126 Overheated The MTD engines are air cooled engines. Because of this, cleanliness of the engine is very important to the life of the engine. Dirt, grass and sludge all form an insulating layer on the engine. This will trap the heat in the engine and cause i...

Failure Analysis 127 Mechanical Breakage/ Wear Sometimes an engine fails because a part breaks. There are generally three causes of a broken part, outside of the previously discussed engine failures. They are abuse, wear, and manufacturing defects. A very common way to abuse an engine is a bent cran...

MTD Products Inc - Product Training and Education Department FORM NUMBER - 769-04951A 10/2010 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com